Controlling the morphology of microgels by ionic stimuli

Bergman, Maxime J.; Pedersen, Jan Skov; Schurtenberger, Peter; Boon, Niels

doi:10.1039/c9sm02170a

Cited by 27 publications

(39 citation statements)

References 46 publications

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“…To this purpose, superresolution microscopy experiments ( 15 ) could be helpful when combined with selective labeling of the surface or even of the initiators, as well as studies where the average bare charge and screening electrostatic conditions of the polymer chains would be systematically varied, as recently done in ref. 52 . Additionally, one could rely on neutron scattering measurements, where differentiations of the molecular components could be achieved by contrast variation ( 38 , 53 , 54 ).…”

Section: Discussionmentioning

confidence: 99%

Two-step deswelling in the Volume Phase Transition of thermoresponsive microgels

Monte

Truzzolillo

Camerin

et al. 2021

Proc. Natl. Acad. Sci. U.S.A.

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Thermoresponsive microgels are one of the most investigated types of soft colloids, thanks to their ability to undergo a Volume Phase Transition (VPT) close to ambient temperature. However, this fundamental phenomenon still lacks a detailed microscopic understanding, particularly regarding the presence and the role of charges in the deswelling process. This is particularly important for the widely used poly(N-isopropylacrylamide)–based microgels, where the constituent monomers are neutral but charged groups arise due to the initiator molecules used in the synthesis. Here, we address this point combining experiments with state-of-the-art simulations to show that the microgel collapse does not happen in a homogeneous fashion, but through a two-step mechanism, entirely attributable to electrostatic effects. The signature of this phenomenon is the emergence of a minimum in the ratio between gyration and hydrodynamic radii at the VPT. Thanks to simulations of microgels with different cross-linker concentrations, charge contents, and charge distributions, we provide evidence that peripheral charges arising from the synthesis are responsible for this behavior and we further build a universal master curve able to predict the two-step deswelling. Our results have direct relevance on fundamental soft condensed matter science and on applications where microgels are involved, ranging from materials to biomedical technologies.

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Section: Discussionmentioning

confidence: 99%

Two-step deswelling in the Volume Phase Transition of thermoresponsive microgels

Monte

Truzzolillo

Camerin

et al. 2021

Proc. Natl. Acad. Sci. U.S.A.

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“…The situation might be even direr for ionic microgels where Monte-Carlo simulations suggest that some of the dangling ends might stretch out to distances much larger than the core radius 92 . In a recent paper, Bergman et al proposed that for ionic poly(NIPAM-co-acrylic acid) microgels, the corona and cross-linked core swelling can be considered as two separate entities 93 . Currently only DLS is able to reliably quantify the corona swelling, by measuring the hydrodynamic radius R H .…”

Section: Characterization Of Microgels With Superresolution Microscopymentioning

confidence: 99%

Pathways and challenges towards a complete characterization of microgels

Scheffold

2020

Nat Commun

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Due to their controlled size, sensitivity to external stimuli, and ease-of-use, microgel colloids are unique building blocks for soft materials made by crosslinking polymers on the micrometer scale. Despite the plethora of work published, many questions about their internal structure, interactions, and phase behavior are still open. The reasons for this lack of understanding are the challenges arising from the small size of the microgel particles, complex pairwise interactions, and their solvent permeability. Here we describe pathways toward a complete understanding of microgel colloids based on recent experimental advances in nanoscale characterization, such as super-resolution microscopy, scattering methods, and modeling. T he term "microgel" usually refers to a cross-linked polymer network of colloidal size that is swollen in a good solvent and has a diameter between~100 nm and several micrometers 1-3 (Fig. 1). Colloidal hydrogels are a common subtype where the continuous phase is water. Often, but not always, these networks exhibit conformational changes in response to changes in their environment, such as the solvent composition or temperature 1,4,5. The sensitivity to external stimuli, such as temperature, pH, or solvent conditions, is an essential feature of many microgels, in particular for the most widely studied type of microgels, based on pNIPAM (poly-N-isopropylacrylamide) and its derivatives 1,6 (Fig. 1b). Here, we consider homogeneous suspensions of these particles at different densities from the dilute to the highly packed regime, as well as microgels adsorbed at surfaces. It is also possible to synthesize larger, millimeter-sized, gel particles 7 , or to study two-dimensional assemblies at an interface 8,9. Additional functions, for example, for chemical transformation or sensing applications 10,11 , can be added using core-shell particles or by decorating the microgels with inorganic nanoparticles 12,13. Microgels are already used in several applications as viscosity modifiers and lubricants 14 , for CO 2 capturing 15 or 3D bioprinting 16 , as biocompatible additives and delivery vehicles 17 , sensors, or stimuli-responsive color-changing systems 18,19. Despite their popularity and widespread application, the internal structure of microgels is not well-understood. During the microgel synthesis, the degrees of freedom for the polymer strands are intentionally reduced by forming covalent cross-links between polymer chains. The chemical crosslinking itself is a nonequilibrium process, and it does not necessarily progress uniformly, which may lead to spatial fluctuations, gradients, and heterogeneities in the microgel particle architecture 20. For a complete understanding of a microgel colloid as a material building block or a carrier, it is imperative to know the spatial distribution of polymer and cross-link densities on the nanoscale. Initially, many studies assumed that microgels could be described simply as spherical, homogeneous gel particles, as discussed in ref. 1. Based on this assumption, t...

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“…Recent research has indicated that hydrodynamic properties are dominated by extended dangling ends, and that these respond to changes in the environment. 28 Indeed, our experimental findings provide evidence that the strongly extended dangling ends also contribute to the interaction potential. Since these effects are not considered in the HY model, we have made an attempt to include such an additional contribution V ramp to the potential based on considerations on repulsions between polyelectrolyte polymer brushes, 29 such that V tot,HYR ( r ) = V H ( r ) + V el ( r ) + V ramp ( r ) with σ ramp corresponds to the diameter of the microgel including counter-ion cloud and ε ramp indicates the strength of the repulsions.…”

Section: Methodsmentioning

confidence: 61%

“…However, in the presence of deionising resin, the dangling ends of the microgel network will extend into the solvent, which can increase the hydrodynamic radius R H by several hundred nanometers compared to the charge neutral case. 6,28,32,33 We obtain the size of the microgels in both conditions, so that we have a direct estimate for the dangling end 'shell' of the ionic microgels.…”

Section: Crosslinker Effect On Dangling End Lengthmentioning

confidence: 99%